1. Field of the Invention
This invention relates to a semiconductor laser array apparatus which produces high output power beams with a 0.degree. phase-shift between the adjacent laser beams.
2. Description of the Prior Art
Semiconductor laser devices which are useful as light sources for optical discs, laser printers, optical measuring systems, etc., must produce high output power. However, conventional semiconductor laser devices having a single waveguide structure can only produce low output power, 60-70 mW at their best, even taking into account their window effects and/or the reflectivity control at their facets. In order to oscillate laser light in a certain array mode (i.e., a 0.degree. phase-shift mode, resulting in a single narrow beam with higher output power), semiconductor laser array devices, in which a plurality of waveguides are fabricated in a parallel manner to achieve an optical phase-coupling between the adjacent waveguides, have been studied. However, the optical phase-shift between the adjacent waveguides of these devices is, indeed, 180.degree., and output power light is emitted in a two-beam fashion having a certain angle therebetween, resulting in a far-field pattern having two peaks. Thus, this laser light cannot be condensed into a diffraction limited spot fashion by means of any known optical lens. In order to use these semiconductor laser array devices as light sources of optical discs, laser printers, etc., they must oscillate lasers in a single array mode and emit output power light with a single narrow beam.
FIGS. 4 and 5 show a conventional semiconductor laser array device, which can be produced as follows: On the (001) plane of a p-GaAs substrate 101, an n.sup.+ -Al.sub.0.1 Ga.sub.0.9 As current blocking layer 102 having a thickness of 0.7 .mu.m and an n-GaAs surface-protective layer 103 having a thickness of 0.1 .mu.m are successively formed by liquid phase epitaxy. Then, three straight channels 108 are formed in a parallel manner through both the surface-protective layer 103 and the current blocking layer 102 into the p-GaAs substrate 101. Each of the channels 108 has a width of 4 .mu.m and a depth of about 1 .mu.m. The distance from the center of one channel to that of the adjacent channel is 5 .mu.m. These channels 108 are disposed at right angles to the (110) plane which corresponds to each of the facets of the device. Then, on the n-GaAs surface-protective layer 103 including the channels 108, a p-Al.sub.0.42 Ga.sub.0.58 As cladding layer 104 having a thickness of 0.2 .mu.m in the portions out of the channels 108, a p- or n-Al.sub.0.14 Ga.sub.0.86 As active layer 105 having a thickness of 0.08 .mu.m, an n-Al.sub.0.42 Ga.sub.0.58 As cladding layer 106 having a thickness of 0.8 .mu.m and an n.sup.+ -GaAs contact layer 107 having a thickness of 1.5 .mu.m are successively formed by liquid phase epitaxy. Since the channels 108 are filled with the p-cladding layer 104, the surface of each of the layers 104, 105, 106 and 107 becomes flat. Then, the upper face of the contact layer 107 and the back face of the substrate 101 are subjected to a vapor deposition treatment with metal materials and then heated to form ohmic contacts thereon made of alloys of the metal materials, followed by cleaving at the (011) plane of the wafer, resulting in a conventional semiconductive laser array device.
The optical field distribution of beams oscillated by the conventional laser array device and the far-field pattern attained by the conventional laser array device are shown in FIGS. 6 and 7, respectively, indicating that the optical phase-shift between the adjacent waveguides is 180.degree..
The reason why the conventional semiconductor laser array device having a plurality of waveguides is operated in a 180.degree. phase-shift mode is that laser light is absorbed by the optical coupling area between the adjacent waveguides, which makes threshold gain of the 180.degree. phase-shift mode significantly low.
The above-mentioned phenomenon that the conventional laser array device is operated in a 180.degree. phase-shift mode can be also explained by reference to FIG. 8, which shows the dependence of the threshold gain of all possible array modes (.nu.=1, 2 and 3) of a triple lasing filament array on the difference in refractive index in the lateral direction. This dependence is obtained by a calculating analysis of the waveguides. It can be also seen from FIG. 8 that the conventional laser array device selectively and stably oscillates laser in a 180.degree. phase-shift mode. As mentioned above, such a 180.degree. phase-shift mode attains a far-field pattern having two peaks, which causes difficulty in condensing the laser light into a diffraction limited spot fashion by means of any known optical lens.
Moreover, the conventional laser array device oscillates laser light in an array mode other than the 0.degree. phase-shift mode and the 180.degree. phase-shift mode, thereby producing output light with a plurality of beams. In addition, two or more array modes are mixed without interference therebetween, thereby producing output light with broad beams.
A semiconductor laser array device having an effective index-guided structure in which optical loss in the optical coupling area is zero has been proposed, which is shown in FIG. 9. The production of this laser array device is as follows: On the (001) plane of an n-GaAs substrate 111, an n-Al.sub.x Ga.sub.1-x As cladding layer 112 having a thickness of 0.8 .mu.m, an n- or p-Al.sub.y Ga.sub.1-y As active layer 113 having a thickness of 0.1 .mu.m, a p-Al.sub.x Ga.sub.1-x As cladding layer 114 having a thickness of 0.8 .mu.m, and a p.sup.+ -GaAs contact layer 115 having a thickness of 0.1 .mu.m are successively formed by a crystal growth technique such as metal organic-chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), liquid phase epitaxy (LPE), or the like. Then, ohmic contacts are formed on both sides of the wafer. Three mesa-stripes 116 are then formed in a parallel manner in the wafer by photolithography and a reactive ion-beam etching technique in such a manner that the portion of the p-cladding layer 114 corresponding to the outside of the mesa-stripes 116 has a thickness of 0.3 .mu.m. Each of the mesa-stripes 116 has an width of 3 .mu.m and a height of 1.5 .mu.m. The distance from the center of one mesa-stripe to that of the adjacent mesa-stripe is 4 .mu.m. These mesa-stripes 116 are disposed in the &lt;110&gt; direction of the substrate 111. Then, cleavage is carried out at the (110) plane of the wafer to form a laser array device 117 having a cavity length of about 250 .mu.m.
The oscillation transverse mode attained by this effective-index guided laser array device is composed of a plurality of array modes. This phenomenon can be explained as follows: This effective-index guided laser array device oscillates lasers in all allowed array modes at the same time because the absorption of light at the optical coupling area does not occur and all of the allowed array modes have the same threshold gain, whereas the laser array device shown in FIG. 4 selectively oscillates laser light in a 180.degree. phase-shift mode because laser light is significantly absorbed by the optical coupling area. The breadth of output beams produced by this laser array device which oscillates laser light in a plurality of array modes is several times that of diffraction limited value, which causes difficulty in practical use of the laser array device.